This invention relates to pressure filters and, more particularly, is concerned with tube pressure filters.
In recent years there have been developed a type of pressure filter, conveniently called tube pressure filters, which are capable of operating at high pressures, for example pressures in excess of about 1000 lbs/in.sup.2, and which can be used to separate particulate solids and liquids which are not separable to the same extent by conventional plate filter presses. Various kinds of tube pressure filter have been described, see for example British Patent Specifications Nos. 907,485; 1,194,676; 1,240,465 and 1,271,494. One kind of tube pressure filter essentially comprises (a) a pair of generally coaxial inner and outer tubular bodies arranged one within the other and adapted to be supported in a generally upright position, (b) an impermeable elastic sleeve disposed within and secured to the outer tubular body, (c) a filter element disposed around and supported by the inner tubular body, (d) first outlet means whereby filtrate which has passed through the filter element can be discharged from the tube pressure filter, and (e) second outlet means whereby solid retained on said filter element can be discharged from the pressure filter, the arrangement being such that in a first operating condition of the tube pressure filter said second outlet means is closed and the tubular bodies cooperate with each other to define a closed annular chamber which is divided into generally coaxial and non-intercommunicating inner and outer compartments by said impermeable elastic sleeve, the inner compartment having an inlet for feed material (comprising a mixture of a liquid and a particulate solid) and the outer compartment having an inlet for a hydraulic fluid under pressure, and in a second operating condition of the tube pressure filter said second outlet means is open to enable the particulate solid to be discharged from the inner compartment. Generally, this kind of tube pressure filter includes means for displacing the tubular bodies axially relative to one another between their first and second operating conditions whereby the inner compartment of said annular chamber is opened, i.e. the second outlet means is opened, so that the filter cake of particulate solid formed on the filter element can be discharged.
The inner tubular body and the filter element supported thereby are constructed and arranged so that, in use, when the tubular bodies are in their first position, liquid from the feed material can be forced through the filter element and through apertures in the inner tubular body while the particulate solid of said feed material is retained on the filter element, the liquid being forced through the filter element by the introduction of a hydraulic fluid under pressure into the outer compartment which hydraulic fluid urges the impermeable elastic sleeve towards the filter element so as to compress the particulate solid in the inner compartment and express liquid therefrom; and when the tubular bodies are in their second position, the chamber of annular cross-section is open and said particulate solid can be discharged from the inner compartment. The filter element of a tube pressure filter usually comprises a sleeve formed from a filter cloth material, advantageously supported on a wire mesh screen which is arranged around the inner tubular body of the tube pressure filter. The inner one of said pair of generally coaxial tubular bodies usually comprises a cylindrical central section, around which cylindrical central section there is disposed and supported the filter element, and upper and lower end sections secured one to each end of said cylindrical central section, each of which end sections includes a portion (which generally cooperates with the adjacent portion of the outer tubular body to form the closed annular chamber when said tubular bodies are in their first operating condition) which is of greater diameter than said cylindrical central section, and each of which end sections has mounted thereon, either on or adjacent to the portion of greater diameter than the cylindrical central section, a fairing which extends around an end of the filter element. Each fairing constitutes means whereby a gradual change of diameter occurs from said portion of greater diameter than the central section to said cylindrical central section, i.e. the outwardly facing surface of the fairing slopes inwardly from the rim of said portion of greater diameter than the central section to said central section. The outwardly facing surface of the fairing can be linear or curved, in the latter case with either a concave or convex profile.
As previously mentioned, in most tube pressure filters the filter element comprises a filter cloth sleeve which fits closely over the inner tubular body. Generally, the filter cloth sleeve is constructed, or tailored, to a shape and size enabling it to be directly fitted without further alteration as a replacement for a filter cloth sleeve which has been damaged or whose interstices have been blocked by solid particles. With some forms of tube pressure filter the fitting (sometimes known as the dressing) of the filter element is rendered more difficult by the fact that the end sections of the inner tubular body have a significantly larger diameter than the cylindrical central section which supports the filter cloth sleeve, so that at least one of the end sections of the inner tubular body has to be removed, with the consequential expenditure of time and effort.